Power unit having self-oscillating series resonance converter

ABSTRACT

A power unit including a self-oscillating series resonance converter which is intended to supply a load, for instance an electric bulb, a battery charger and/or electronic equipment. The series resonance converter includes two mutually connected transistors, each of which is activated by a respective control transformer, and further includes an inductor and a capacitor which are connected in series. Each transistor and an associated series-connected capacitive voltage divider and/or a further transistor are connected in parallel with the inductor, capacitor and control transformer. The load is connected across the capacitor. The magnetic flux through the control transformer can be influenced by a magnetic flux applied externally via an electromagnet to change the oscillation frequency of the transistors and therewith the voltage output of the power unit.

BACKGROUND OF THE INVENTION

The present invention relates to a power unit which includes aself-oscillating series-resonance converter and which functions tosupply a load, such as an electric bulb, a battery charger and/orelectronic equipment, wherein the series-resonance converter includestwo mutually connected transistors, each of which is intended to beactivated by a control transformer, and a series-connected inductor andcapacitor, and wherein each transistor and a correspondingseries-connected capacitive voltage divider belonging to the transistorand/or a further transistor is connected in parallel with the inductor,capacitor and control transformer, and wherein the load is connectedacross the capacitor.

A power unit of the aforedescribed kind is known to the art and is founddescribed, for instance, in Swedish Patent Specification No. 7512267-1.This power unit operates in accordance with the series-resonanceprinciple and provides many advantages over other known power units.These advantages include low switch losses at high operatingfrequencies, while problems relating to radio interference ordisturbance are negligible.

One of the drawbacks with a power unit that operates in accordance withthe series-resonance principle is that it requires the presence ofcomplicated drive electronics. This results in a high total cost for themanufacture of such a power unit. Since there is a high demand for powerunits which can be used in low-price equipment, it is desirable to makethe drive circuits of the power unit as cheap as possible.

SUMMARY OF THE INVENTION

A power unit including a self-oscillating series resonance converterwhich is intended to supply a load, for instance an electric bulb, abattery charger and/or electronic equipment. The series resonanceconverter includes two mutually connected transistors, each of which isactivated by a respective control transformer, and further includes aninductor and a capacitor which are connected in series. Each transistorand an associated series-connected capacitive voltage divider and/or afurther transistor are connected in parallel with the inductor,capacitor and control transformer. The load is connected across thecapacitor. The magnetic flux through the control transformer can beinfluenced by a magnetic flux applied externally via an electromagnet tochange the oscillation frequency of the transistors and therewith thevoltage output of the power unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawings, wherein:

FIG. 1 is a circuit diagram for a power unit operating in accordancewith the series-resonance principle.

FIG. 2 is a diagram which illustrates a voltage/current relationship inthe power unit.

FIG. 3 is a circuit diagram for the power unit according to FIG. 1, saidcircuit enabling the magnetic flux through the control transformerbelonging to the power unit to be regulated or adjusted.

FIG. 4 is a view from above of the arrangement of a power unit controltransformer having an electromagnet connected externally thereto inaccordance with the principles of the invention.

FIG. 5 is a circuit diagram which relates to a modified version of thepower unit illustrated in FIG. 3.

DETAILED DESCRIPTION

The manner of operation of the power unit illustrated in FIG. 1 will nowbe described.

When the power unit is connected to the mains voltage, which in thepresent case reaches an alternating voltage of 220 volts, the voltage isrectified with the aid of four bridge-connected rectifier diodes D1-D4.The full-wave rectified mains voltage is filtered downstream of thediode bridge with the aid of a first capacitor C1. The direct voltageacross the capacitor C1 will reach about 310 volts. A current whichcharges a capacitor C2 now passes through a resistor R1. When thevoltage across this latter capacitor C2 has risen to above the thresholdvoltage of a trigger diode D5, the trigger diode is ignited or fired andconducts current to the base of a first transistor TR1. The purpose ofthe illustrated diode D6 is to prevent triggering of the trigger diodeD5 when the circuit self-oscillates. The transistor TR1 is nowconductive and a capacitor C3 is discharged via a further resistor. Thevoltage across the transistor TR1 will reach about 310 volts before thetransistor becomes is turned on. When the transistor TR1 is turned on,current passes through the transformer T1, the inductor L1 and thecapacitor C4. The current passing through the primary winding P of thetransformer T1 gives rise to magnetic flux in the core K of thetransformer T1. In turn, this flux induces a voltage in a secondarywinding S1 of the transformer T1, which causes the first transistor TR1to become conductive. The current passing through the transformer T1will increase while the first transistor TR1 is operative. The core K ofthe transformer T1 becomes saturated after a given length of time, asillustrated in the FIG. 2 diagram. This causes the first transistor TR1to cease to conduct. Because of the back electromotive force generatedby the inductor L1, the voltage in the common contact point A of thefirst transistor TR1 and of a second transistor TR2 will be convertedfrom low potential to high potential. The current passing through thetransformer T1 thereafter changes direction, causing the earlierdescribed sequence to be repeated. In turn, this causes the transistorsTR1, TR2 to conduct alternately. When the components included aredimensioned correctly, the switching frequency of the circuit concernedwill normally be higher than the resonance frequency of the inductor L1and the capacitor C4. The voltage across the capacitor C4 is determinedby how close the resonance frequency of the inductor L1 and thecapacitor C4 lies to the switching frequency. As illustrated in thedrawings, a load can be connected across the capacitor C4. The diodesD9-D10 are antiparallel to the transistors TR1 and TR2.

The illustrated arrangement thus provides an extremely simpleconstruction. However, this construction has a serious drawback, namelythat it is not possible to adjust or control the output voltage in asimple manner. The invention provides an arrangement which with the aidof an external magnetic field enables the core K of the transformer T1to be saturated rapidly when the external magnetic field increases, andto be saturated more slowly when the external magnetic field decreases.This is achieved with the power unit illustrated in FIG. 3, which is adevelopment of the arrangement illustrated in FIG. 1 and which operatesin accordance with this principle. The voltage across the capacitor C4is dependent on the oscillation frequency of the transistors TR1, TR2.Connected across the capacitor C4 is a transformer T2 which transformsthe capacitor voltage to a suitable level. The voltage is rectified bythe diodes D7, D8. The rectified voltage is then filtered by means ofthe inductor L2 and the capacitor C5. When the output voltage increases,so that the zener diode Z1 placed in the winding circuit of theelectromagnetic T3 begins to conduct, current will pass through thezener diode and also through the winding W1 of the illustratedelectromagnet T3. The current passing through the winding W1 gives riseto a magnetic flux which hastens saturation of the core K of thetransformer T1, which in turn causes the oscillation frequency toincrease and distance itself from the resonance frequency of theinductor L1 and the capacitor C4. As a result, the voltage across thecapacitor C4 becomes lower, as does also the power unit output voltage.There is thus obtained a power unit having a controlled or adjustedoutput voltage.

FIG. 4 illustrates a suitable embodiment of the transformer and of thetransformer magnetizing arrangement. The transformer T1 operates as atypical ferrite transformer, in which the primary winding P induces analternating flux in the ring core K. In turn, the flux induced in thering core K induces a voltage across the secondary windings S1, S2. Theoscillation frequency is now governed by the speed at which the ringcore K of the transformer T1 becomes saturated; see also FIG. 2.Application of an externally arriving flux in addition to the fluxarriving from the primary winding P of the transformer T will cause thering core K of the transformer T1 to be saturated more rapidly. Thus,the oscillation frequency increases. The magnetic flux inducedexternally via the electromagnetic T3 in the ring core K is added to theflux from the primary winding P of the transformer T1. The ring core Kof the transformer T1 can thus be brought to saturation more or lessquickly, by changing the current in the winding W1 of the electromagnetT3.

The electromagnet T3 used in the illustrated arrangement includes aniron yoke O manufactured from transformer plate in a known manner. Asbefore mentioned, magnetization of the yoke O through the medium of thecurrent in the winding P of the electromagnet T3 will also change theflux in the ring core K of the transformer T1. The iron yoke O ismounted externally of the ring core K with a connection transverselyacross the core, between the centre point of the primary winding P andthe space between the secondary windings S1, S2.

The arrangement illustrated in FIG. 2 relates to a so-called halfbridge, wherein capacitive voltage dividers C6, C7 are connected inparallel with respective transistors TR1, TR2. Naturally, the voltagedividers C6, C7 can be replaced with transistors TR3, TR4, so as toobtain a so-called full bridge according to FIG. 5.

The winding circuit of the electromagnet T3 is connected to the powerunit output in both the arrangement illustrated in FIG. 3 and thearrangement illustrated in FIG. 5.

I claim:
 1. A power unit which includes a self-oscillating seriesresonance converter and which is intended to supply a load, wherein theseries resonance converter includes:two transistors connected to oneanother, each of which is activated by a respective control transformer;and an inductor and a capacitor which are connected in series; each saidtransistor having an associated series-connected capacitive voltagedivider or a further transistor connected in parallel with saidinductor, capacitor and control transformer; a load connected across thecapacitor; magnetic flux through the control transformer arranged to beinfluenced by a magnetic flux applied externally via an electromagnet tochange the oscillation frequency of the transistors and therewith thevoltage output of the power unit.
 2. The power unit according to claim1, wherein:the electromagnet has a winding circuit, and current throughthe winding circuit of the electromagnet is arranged to be controlled bythe voltage output of the power unit.
 3. The power unit according toclaim 2, wherein:said winding circuit includes a zener diode fordetecting output voltage.
 4. The power unit according to claim 1,wherein:said control transformer includes a ferrite ring-core.
 5. Thepower unit according to claim 4, wherein:in addition to a primarywinding having a center point, said control transformer also includestwo secondary windings having the space between them, with connectionsto the bases of the respective ones of said transistors.
 6. The powerunit according to claim 5, wherein:said electromagnet has a yoke and aring core; said yoke being comprised of a transformer plate, and isconnected across the ring core (K) in the vicinity of said center pointof said primary winding, and said space between said secondary windings.